Among known bleaching agents, hydrogen peroxide is at the present time being increasingly used, in particular for bleaching materials such as textiles or paper pulp. Hydrogen peroxide has the great advantage over other bleaching agents, in particular chlorine and its compounds, in that because of its mild action, it attacks the fibers of the material to be treated to a much lesser extent, while exerting a more durable action and giving a better finish.
Hydrogen peroxide is generally used in bleaching in the form of a stabilized alkaline solution of low peroxide concentration. The action of the hydrogen peroxide in bleaching consists essentially of destroying or decolourising the natural dyes by oxidation, or by rendering them soluble. Even though the mechanism of these reactions has still been little studied, it is generally assumed that the hydrogen peroxide ion HOO.sup.- is responsible for the bleaching.
Present-day bleaching solutions based on hydrogen peroxide have however the great disadvantage with respect to other conventional bleaching solutions (in particular hypochlorite-based solutions) of being relatively costly, so that their widespread use is very dependent on economic considerations, in particular when large quantities of low-value material such as paper pulp are to be treated. Present-day bleaching solutions are nearly always prepared by simple dissolving and dilution, starting from commercially available chemicals. Commercially available hydrogen peroxide is a particularly costly substance, as it is manufactured only in a small number of large industrial plants, and it has therefore to be highly concentrated for storage and transport purposes before being distributed.
At the present time there is a need to replace these preparation methods using highly concentrated commercially available constituents, by in situ manufacturing methods which enable dilute solutions of hydrogen peroxide to be produced directly, in order to reduce bleaching costs. However, up to the present time no satisfactory method has appeared.
Hydrogen peroxide is used not only for bleaching purposes, but also in an increasing number of other processes, in particular in the pollution control field. However, treatment solutions used for this purpose are likewise almost always prepared from highly concentrated commercial constituents, and they thus have the same disadvantages as heretofore stated.
It has been proposed for some time to prepare hydrogen peroxide electrolytically, by reducing oxygen at a cathode in an alkaline medium. However, the methods proposed up to the present time all aim at large-scale production of hydrogen peroxide, so that the products obtained by these methods are much too concentrated for direct use as treatment solutions (the alkaline solutions used in these methods have a very high initial concentration).